Polyethylene terephthalate film, process for the production thereof and magnetic recording medium therefrom

ABSTRACT

The present invention relates to a polyethylene terephthalate film having the sum of F-5 values in longitudinal and transverse directions of 3500 to 5000 kg/cm 2 , absolute heat shrinkages in longitudinal and transverse directions at 100° C. of up to 2.5% and amorphous orientation factor of -1.5 to 0.5, a process for the production thereof and a magnetic recording medium containing the film as a base. The polyethylene terephthalate film useful as a magnetic recording medium having super high-tensile-strength in both longitudinal and transverse directions and a low heat shrinkage is provided, while it has been considered that such properties could not be obtained in polyethylene terephthalate film.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a polyethylene terephthalate(hereinafter referred to as PET) film, process for the productionthereof and magnetic recording medium containing the same as a base.Particularly, the present invention relates to a PET film having quitehigh mechanical strengths, both in longitudinal and transversedirections, excellent dimensional stability and low heat shrinkage,process for the stable production thereof and the magnetic recordingmaterial such as a magnetic tape containing this film as a base.

(2) Description of the Prior Art

As for PET films having high mechanical strengths, both in longitudinaland transverse directions, and process for the production thereof, therehas been known a process wherein a biaxially oriented film issuccessively restretched biaxially in longitudinal and transversedirections as disclosed in the specification of U.S. Pat. No. 4,226,826.However, this process has defects that the mechanical strength, i.e.,F-5 value of the film cannot be increased sufficiently and if thestretching ratio is increased so as to obtain a film having a high F-5value, the restretchability becomes bad and heat shrinkage is increased.Further, there has been known a process wherein a biaxially orientedfilm is simultaneously restretched both longitudinally and transverselyas shown in the specification of U.S. Pat. No. 4,234,532. However, thisprocess also has defects that if the F-5 value is increased, therestretchability becomes bad to cause troubles such as breakage of thefilm in the process of the stretching of the film and heat shrinkage isincreased. Therefore, when a film obtained by either of these knowntechniques is used as a base to provide a magnetic recording layerthereon, the base film is distorted or a uniform tone of an audiorecording type or picture quality of a video tape cannot be obtained.

It has been impossible in the prior art to obtain a film having bothsatisfactory mechanical strength and dimensional stability. One of thecharacteristic feature of the present invention is that the longitudinalstretching which is effected first is carried out under specificconditions to obtain a stretched film having specific properties,particularly birefringence and amorphous orientation factor and,accordingly, restretchability in the next transverse stretching and/orrestretching step is improved and properties of the film thus obtainedare far superior to those of films obtained in the prior art. Thus, theinventors have succeeded in obtaining a film having quite highstrengths, and excellent dimensional stability such as low heatshrinkage.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a PET film having highmechanical strengths in both longitudinal and transverse directions anda high dimensional stability. Another object of the present invention isto provide a PET film for the production of a magnetic recording mediumhaving excellent mechanical properties such as a high F-5 value and alow heat shrinkage. Still another object of the present invention is toprovide a process for producing a PET film having the above-mentionedexcellent properties. A further object of the invention is to provide amagnetic recording medium containing the PET film as a base. Anotherobject of the invention is to provide a magnetic recording tape capableof elongated recording and free of scalloping due to curling. Anotherobject of the invention is to provide a super tensilized PET film havingwell-balanced strengths in both longitudinal and transverse directions,with high mechanical strengths in both directions and with excellentdimensional stability.

Namely, the present invention relates to:

(1) A super tensilized high-tenacity PET film having the sum of F-5values in both longitudinal and transverse directions of 3500 to 5000kg/cm², absolute heat shrinkage in longitudinal and transversedirections of up to 2.5% at 100° C. and an amorphous orientation factorof -1.5 to 0.5,

(2) a process for producing an oriented PET film which compriseslongitudinally stretching an unoriented PET film in multiple stages toobtain an amorphous orientation factor of up to 0.6 to 1.0 andbirefringence of 0.02 to 0.1, then stretching the same transversely 2.5to 4.5-fold and restretching the same biaxially at a stretching ratio of1.5 to 2.5 in each direction to attain a total stretching ratio of atleast 27, and

(3) a magnetic recording film containing this film as a base and amagnetic layer formed thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

PET according to the present invention is ethylene terephthalatehomopolymer or polyethylene terephthalate containing up to 20% of athird component. The ethylene terephthalate homopolymer is particularlypreferred. The polyethylene terephthalate is synthesized by reactingterephthalic acid or a functional derivative thereof with ethyleneglycol or a functional derivative thereof in the presence of a catalystunder proper reaction conditions. One or more third component(s) may beadded to the reaction system before or after completion of thepolymerization of ethylene terephthalate to obtain a copolymer or mixedpolyester.

As the copolymerizable components, there may be used generally compoundscontaining an ester-forming functional group such as a dicarboxylicacid, e.g., isophthalic acid, naphthalenedicarboxylic acid,4,4'-biphenyldicarboxylic acid, α,β-bis(phenoxy)ethane-4,4'-dicarboxylicacid, α,β-bis(2,6-dichlorophenoxy)ethane-4,4'-dicarboxylic acid andα,β-bis(2-chlorophenoxy)ethane-4,4'-dicarboxylic acid and dihydroxycompounds, e.g., 1,4-cyclohexanedimethanol and propylene glycol. PET maycontain also internal particles or external particles as a lubricant. Bythe term "internal particles" as used herein is meant particles whichare precipitated in the polyester because at least part of the catalystresidue or anti-coloring agent, containing Ca, Si, Mn, Mg, Sb, Ge, P.Li, K or Na, is reacted with a monomer or oligomer in thepolycondensation stage thereby forming insoluble particles. Suchinternal particles are described in U.S. Pat. Nos. 4,138,386 and4,067,855.

The internal particles have an average particle diameter of 0.1 to 5μ ingeneral. The term "external particles" herein means chemically inactiveparticles selected from the group consisting of oxides and inorganicsalts of elements of Groups II, III and IV in the Periodic Table havingan average particle diameter of 0.1 to 10μ. They are, for example,titanium oxide, fine silica particles and calcium carbonate. The PET maycontain a stabilizer such as phosphoric acid, phosphorous acid or anester thereof. PET has an intrinsic viscosity of preferably 0.4 to 1.0,particularly 0.55 to 0.8. The preferred range of the intrinsic viscosityis the same as that of the final film product.

The film of the present invention is characterized in that it has hightensile-strength in both longitudinal and transverse directions.Indispensable conditions of this film are as follows:

The PET film of the present invention has the sum of F-5 values inlongitudinal and transverse directions of 3500 to 5000 kg/cm². If thesum is less than 3500 kg/cm², it is difficult to produce the magneticrecording medium recording for a long period of time and when such afilm is used as a base film of a thin video magnetic recording mediumhaving a thickness of up to 9μ, it exhibits an insufficient toughnessand dimensional stability against an external force. Thus, it cannot beused as a magnetic recording medium of which a high precision isrequired. It is thus desirable that the sum of F-5 values inlongitudinal and transverse directions is above 3500 kg/cm², and at thesame time the F-5 value in longitudinal direction is at least 1800kg/cm² and that in transverse direction is at least 1700 kg/cm².

The sum of F-5 values exceeding 5000 kg/cm² brings about the lowering ofproductivity such as deterioration of the flatness.

In case the film is used as a magnetic recording medium for a smallerdevice, such as a video cassette type, a preferable F-5 value in thelongitudinal direction is 2000 kg/cm² or higher.

The film of the present invention has absolute heat shrinkages in bothlongitudinal and transverse directions at 100° C. of up to 2.5%. If thefilm has a higher heat shrinkage, its dimensional stability is reducedby the influence of heat in the magnetic layer-forming step.

The film of the present invention has an amorphous orientation factor of-1.5 to 0.5. If this factor is below -1.5, the resulting film has anunbalanced tensile strength, i.e., a higher transverse tensile strength.On the contrary, if it exceeds 0.5, the resulting film has an unbalancedtensile strength, i.e., a higher longitudinal tenacity.

The polyethylene terephthalate constituting the film has an averagecrystal size of preferably up to 50 Å, particularly up to 45 Å. If theaverage crystal size is below 50 Å, the longitudinal and transversetensile strength are well-balanced and heat shrinkage can be reducedeasily. The lower the minimum average crystal size, the better. Thepossible minimum value is generally 30 Å, though it is not particularlylimited.

Now, the description will be made on the process for producing the PETfilm of the present invention.

The above-mentioned PET is molten and extruded through a nozzle or a dieslit in an ordinary manner to form a sheet, which is then cast on acooling drum and solidified. The melting temperature is generally 260°to 300° C. and the cooling drum is preferably cooled at 5° to 75° C. Inthe course of the extrusion and casting, it is effective for example toapply an electrostatic charge so as to prevent the involution of airbetween the cooling drum and the cast sheet or the deposition ofmonomers and oligomers on the cooling drum. The obtained cast sheet isthen stretched longitudinally in multiple stages. It is necessary in thepresent invention that the longitudinally stretched film has specifiedcharacteristics. Therefore, the multi-stage longitudinal stretching iscarried out under specific conditions. Namely, the longitudinallystretched film should have amorphous orientation factor of 0.6 to 1.0exclusive and birefringence of 0.02 to 0.1, preferably 0.03 to 0.08. Themulti-stage longitudinal stretching is performed by a method based onthe difference in circumferential velocity of rollers.

Recommended conditions in the multi-stage stretching step in the presentinvention will be shown as follows. In the first stage, the stretchingtemperature is 110° to 150° C., preferably 115° to 135° C. andstretching ratio is 1.3 to 3.0, preferably 1.5 to 2.5. In and after thesecond stage, the stretching temperature is 85° to 120° C., preferably95° to 115° C. and is lower than that of the first stage and stretchingratio is 2.0 to 4.0, preferably 2.0 to 3.0.

If the temperature in the first stage and/or that in the second stageare (is) below the above-mentioned ranges, the birefringence wouldexceed 0.1 unfavorably. On the other hand, if the temperature in thefirst stage and/or that in the second stage are (is) above theabove-mentioned range, the amorphous orientation factor would be below0.6.

If the amorphous orientation factor and birefringence are beyond theabove-mentioned ranges, the film would be broken in the subsequenttransverse stretching step and re-stretching step, whereby the operationis interrupted each time. As a result, the stable production of the filmbecomes impossible. Further, the resulting film may have insufficient ornonuniform physical properties and quality.

A reason why the first multi-stage longitudinal stretching is effectivefor obtaining the film of the present invention having specificproperties has not fully been elucidated yet. It is supposed, however,that an amorphous part can be highly oriented, though thecrystallization does not proceed, by the multi-stage longitudinalstretching under the specific conditions. It is considered that thisfact greatly contributes to the production of a tensile-strength filmhaving a low coefficient of shrinkage without the breakage.

The transverse stretching after the multi-stage longitudinal stretchingstep is carried out generally by means of a tenter wherein both ends ofthe film is clipped. The transverse stretching ratio is at least 2.5,preferably 2.6 to 4.5. The stretching temperature is in the range from apoint 5° C. higher than the second-order transition temperature of thefilm to 150° C., preferably, from a point 10° C. higher than thesecond-order transition temperature of the film to 130° C.

The biaxial stretching after the transverse stretching may be carriedout by either longitudinal→transverse successive biaxial stretching(hereinafter referred to as "successive biaxial restretching") orsimultaneous longitudinal/transverse biaxial stretching (hereinafterreferred to as "simultaneous biaxial stretching"). Of these two types ofbiaxial restretching, the latter is preferable because of ease inattaining well-balanced longitudinal and transverse strengths.

Preferable modes of embodiment of these biaxial restretchings will nowbe described.

(1) Successive biaxial restretching method: In this method, a film islongitudinally stretched by an ordinary roll method and thentransversely stretched by an ordinary tenter method. The film to bestretched may be in any desired form such as a film with edges, edgelessfilm or film with turned up edges.

The stretching ratio in each of the longitudinal and transversedirections is 1.5 to 2.5, preferably 1.5 to 2.3 and the stretchingtemperature is in the range from a point 10° C. higher than thesecond-order transition temperature to a point 40° C. lower than themelting point, preferably in the range from a point 15° C. higher thanthe second-order transition temperature at 170° C. These conditions aresuitable for obtaining a film having good stretchability andwell-balanced tensile strength in both directions.

(2) Simultaneous biaxial stretching method: As this method, any processcapable of stretching a film in both longitudinal and transversedirections simultaneously may be employed. For example, both edges of afilm are clipped and distances between the clips along longitudinaldirections and between the clips in transverse direction are extendedsimultaneously to effect the stretching in both longitudinal andtransverse directions simultaneously. The film to be clipped may haveany desired form such as a film with edges, edgeless film or film withturned up edges. The simultaneous biaxial stretching is effected at astretching ratio of 1.5 to 2.5, particularly 1.5 to 2.3, in each oflongitudinal and transverse directions at a stretching temperature inthe range from a point 10° C. higher than the second-order transitiontemperature to a point 40° C. lower than the melting point, particularlypreferably from a point 15° C. higher than the second-order transitiontemperature to 170° C.

Reasons why the above-mentioned conditions are preferred in thesimultaneous biaxial stretching method are that within this temperaturerange, a film having a high tensile-strength can be obtained easily,frequent breakage of the film can be prevented, and loss of transversetensile strength and unevenness of the quality due to transverseneck-down between the clips can be prevented. The stretching ratios inthe longitudinal and transverse directions may be either equal orunequal.

The product of the stretching ratio (total-area stretching ratio) in allthe above-mentioned steps including the multi-stage longitudinalstretching, transverse stretching and biaxial restretching steps shouldbe at least 27, preferably at least 28 and particularly at least 30. Ifthis value is less than 27, the intended high-tensile-strength filmhaving well-balanced longitudinal and transverse strengths cannot beobtained.

The film thus restretched under the above-mentioned conditions is thensubjected to heat setting to further increase its dimensional stability.

The heat setting is preferably carried out at a temperature in the rangefrom a point 50° C. higher than the second-order transition temperatureto a point 10° C. lower than the melting point, particularly preferablyat least a point 10° C. higher than the biaxial restretching temperatureand in the range from a point 80° C. higher than the second-ordertransition temperature to a point 20° C. lower than the melting pointunder tension or particularly preferably under relaxation. Thistreatment may be effected by rolls or a tenter.

By this heat setting, a film having absolute heat shrinkage of up to2.5% in each of the longitudinal and transverse directions at 100° C.can be obtained in the present invention.

The film thus heat-set may be subjected to a known corona dischagetreatment in an atmosphere of nitrogen, air, carbon dioxide or a mixtureof two or more of them. Further, a coating or a laminate may be formedon the film so as to impart adhesiveness, wet resistance, heatsealability, lubricity and surface smoothness thereto. The thickness ofthe PET film of the present invention is not particularly limited but athickness in the range of 1 to 75μ is preferred for a magnetic recordingtape or magnetic recording disc.

The PET film thus formed is substantially free of longitudinal wrinkles,etc. in a step of winding to form a roll and the roll has a goodappearance. In addition, the film has both well-balanced andlongitudinal and transverse tensile strength and a dimensional stabilityfar superior to those of the conventional products.

As described above in detail, according to the present invention, aneffect is achieved that a PET film having high longitudinal andtransverse tensile strength and dimensional stability can be obtainedstably by employing a combination of (1) specific biaxial stretchingcomprising specific multi-stage longitudinal stretching and transversestretching with (2) biaxial re-stretching.

The film thus obtained according to the present invention is suitablefor use as a quite thin film such as a base of magnetic recordingmedium, dielectric of condenser and graphic and/or printing material.The film is suitable particularly for use as a base film of magneticrecording medium.

Then, a magnetic layer is formed on at least one surface of theabove-mentioned film according to the present invention to form amagnetic recording medium.

The magnetic layer of the present invention may by (1) a known magneticlayer formed by the so-called coating application method wherein adispersion of a powdery magnetic material such as Co-doped γ-Fe₂ O₃,γ-Fe₂ O₃, CrO₂ or a ferromagnetic alloy in an organic binder is used or(2) any ferromagnetic layer formed by vacuum deposition, sputtering,ion-plating or electro-plating method wherein a ferromagnetic metal suchas Fe, Co or Ni or a magnetic alloy containing the same is used. Thelatter ferromagnetic layer is preferred. The thickness of the magneticlayer which varies depending on the magnetic layer-forming method isgenerally preferably in the range of 0.05 to 5μ.

In case the recording is to be effected over a long period of time andparticularly when a ferromagnetic layer is used, it is preferred tocontrol the total thickness of the magnetic recording medium includingthe magnetic layer with the range of 4 to 9μ.

In the binder-type magnetic tape prepared by applying a magnetic coatingmainly comprising a magnetic powder and a binder to a base film, it isnecessary that the base film should have a high dimensional stability ata high temperature, since the temperature is elevated in a step ofdrying of the binder or the roll-press and also due to the elevation ofthe processing temperature caused by an increase of the processingvelocity.

For a so-called metallized magnetic recording medium prepared by thedirect vacuum metallizing of a ferromagnetic substrate, a base filmshould have a high dimensional stability at a high temperature becausethe temperature of the base is kept at 80° C. or higher in themetallizing preparation step thereof.

If the base film has high heat shrinkage, the strain in theabove-mentioned treatment is increased and developed during the actualuse thereof as a magnetic recording medium under severe conditions.

A film used as a magnetic recording medium, particularly video cassettetape, should have a high mechanical strength and also maintain stablerunning properties for a long time, since it is subjected to repeatedrunning and loading under severe conditions. If these conditions are notsatisfied, an image disorder is caused at the time of starting orstopping or during running.

As a video camera has become smaller recently, reduction of the size andelongation of the recording time are required also of a cassette tapeused. Further, a reduction in thickness of both the magnetic layer andthe base film is demanded. However, a thin base film has high heatshrinkage and low dimensional stability unfavorably.

The magnetic recording medium containing the film of the presentinvention is free of these defects.

Characteristic properties such as F-5 value, heat shrinkage, amorphousorientation factor and average crystal size of the film layer of theresulting magnetic recording medium are substantially equal to those ofthe film before the formation of the magnetic layer.

The magnetic recording medium of the present invention may be used as amagnetic tape, magnetic card or magnetic disc in a computer, an audio orvideo set or a measuring device. It is particularly suitably used in avideo set.

The amorphous orientation factor, F-5 value, heat shrinkage, averagecrystal size, birefringence, melting point and second-order transitiontemperature in the present invention were determined by the followingmethods:

(1) Amorphous orientation factor:

A polyester film was immersed in a water bath containing opticalwhitening agent "Mikephor ETN" (a product of Mitsui Toatsu ChemicalsInc.) at 55° C. and then dried with air. A parallel component ofpolarized fluorescence intensity in the film plane was measured by meansof a polarizing fluorometer "FOM-1" (a product of Japan SpectroscopicCo., Ltd.). The amorphous orientation factor (F) was determinedaccording to the formula:

    F=1-(B/A)

wherein:

F represents amorphous orientation factor,

A represents a parallel component of polarized fluorescence intensity inlongitudinal direction, and

B represents a parallel component of polarized fluorescence intensity ina transverse direction.

(2) F-5 value:

A film cut into a width of 10 mm (when it had a magnetic layer formed onthe film, the layer was removed before the test) was set in such amanner that a distance between chucks was 100 mm. A tensile strength ofthe film at 5% stretching was measured at an extension rate of 20 mm/minat 25° C.

(3) Heat shrinkage:

The length of a sample was measured. Then, the sample was left to standin an air-conditioned oven kept at 100° C. under no tension for 30minutes. After this heat treatment followed by cooling, the length ofthe sample was measured again. The heat shrinkage was calculated fromthe lengths before and after the heat treatment.

(4) Average crystal particle size:

A half-height width of a [100] crystal plane was determined with anX-ray diffraction device (a product of Rigaku Denki Co., Ltd.). Theaverage crystal size was calculated according to the following formula:

    A=(0.9λ/B cos θ)

wherein:

λ is 2.2896 (Å) and

B represents a half-height width (radian).

As a base line, a straight line connecting points having scatteringintensities of 2θ=50° and 2θ=20° was employed.

(5) Birefringence:

Birefringence was measured at a temperature of 25° C. and a relativehumidity of 65% after taking a sample by means of a Berek compensator.

(6) Melting point and second-order transition temperature:

Melting point and second-order transition temperature were measuredaccording to ASTM D 3418-75.

The following examples will further illustrate the embodiments of thepresent invention.

EXAMPLES 1-5

Polyethylene terephthalate of IV 0.65 was melt-extruded at 290° C. andthen cooled rapidly to obtain a substantially amorphous sheet. The sheetwas stretched longitudinally taking advantage of a circumferential speeddifference of multi-stage nip rollers at 125° C. at a stretching ratioof 2.05 in the first stage and continuously stretched longitudinally at113° C. at a stretching ratio of 2.32 in the second stage. The resultingmulti-stage longitudinally stretched film had an amorphous orientationfactor of 0.81, birefringence of 0.038 and density of 1.3442.

Then, the film was stretched transversely in a tenter at 105° C. at astretching ratio of 2.8. The film was further stretched at a stretchingratio as shown in Table 1-1 in a simultaneous biaxial stretching tenterat 150° C. at a rate of 3000 %/min and then heat-set at 200° C. undertension. The final thickness of the film was 7 to 9μ. Characteristicproperties of the resulting films were examined to obtain the resultsshown in Table 1-2.

As shown in Examples 1-3 in Table 1-2, if simultaneous stretching ratiois increased while longitudinal and transverse stretching ratios arekept constant, F-5 values in both longitudinal and transverse directionsare improved and a super tensilized film having the sum of F-5 values inlongitudinal and transverse directions of 3500 kg/cm² can be obtained.Heat shrinkage after heating at 100° C. for 30 min was not higher than2.5%. The total area stretching ratio was at least 30 in all cases. Thestretching at such a high stretching ratio can thus be attained withoutbreakage. The stretchability was excellent.

In Example 4, the transverse stretching ratio was higher than thelongitudinal stretching ratio in the simultaneous biaxial stretchingstep to effect slightly unbalanced stretching. And also in Example 5,the longitudinal stretching ratio was slightly higher than thetransverse stretching ratio in the simultaneous biaxial stretching step.

EXAMPLES 6-11

Films were stretched in the same manner as in Examples 1-5 except thatthe longitudinal stretching conditions in the first and second stageswere altered as shown in Table 1-1 to obtain longitudinally stretchedfilms having various amorphous orientation factors and birefringences.Then, the films were stretched transversely under conditions shown inTable 1-1 and then subjected to the simultaneous biaxial stretchingunder conditions shown in Table 1-1 to obtain final films. All the filmshad a high stretchability and a total area stretching ratio of at least30.

As for characteristic properties of the resulting films, F-5 values inlongitudinal and transverse directions and simultaneous biaxialstretching ratios were increased as shown in Table 1-2. The sum of F-5values in longitudinal and transverse directions of at least 3500 kg/cm²were obtained. Heat shrinkages after heating at 100° C. for 30 min wasup to 2.5%.

COMPARATIVE EXAMPLES 1-3

In these comparative examples, the longitudinal stretching, transversestretching and longitudinal restretching were carried out. The firstlongitudinal stretching was carried out in one stage. Stretchingconditions were as shown in Table 1-1. In Comparative Example 1, thelongitudinal stretching was effected at a low temperature at a highstretching ratio at once. The film thus stretched longitudinally hadbirefringence of above 0.1 and poor stretchability. The film was brokenfrequently at a stretching ratio of above 1.5. Accordingly, satisfactoryhigh-tensile-strength film could not be obtained. In Comparative Example2, the stretching ratio in the first longitudinal stretching step waskept low and that in the simultaneous biaxial stretching step wasincreased. The resulting film had a low amorphous orientation factor anda low mechanical strength. In Comparative Example 3, the stretching inthe first step was carried out at a high temperature and a highstretching ratio. The resulting film had a low amorphous orientationfactor, a low birefringence and a low mechanical strength. In all cases,the films were broken frequently and the stable production ofhigh-strength films was impossible.

COMPARATIVE EXAMPLES 4-6

In these comparative examples, the mutli-stage longitudinal stretching,transverse stretching and simultaneous biaxial stretching were carriedout under stretching conditions as shown in Table 1-1. The stretchingconditions were unsuitable and, therefore, physical properties of thefilms stretched longitudinally in multiple stages did not satisfy therequirements according to the present invention. The films showed poorstretchability in the subsequent stretching steps. F-5 values and heatshrinkages of the resulting films were also unsatisfactory.

                                      TABLE 1-1                                   __________________________________________________________________________                                            Simultaneous                                               Characteristics of Biaxial                                                    films after multiple-                                                                            Stretching                            Multiple Longitudinal                                                                              stage longitudinal                                                                       Transverse                                                                            Conditions  Total                     Stretching Conditions                                                                              Stretching Stretching  Longi-                                                                            Trans-                                                                            Stretching                1st Stage    2nd Stage                                                                             Amorphous  Conditions  tudinal                                                                           verse                                                                             Ratio (Time)              Temp.    Ratio                                                                             Temp.                                                                             Ratio                                                                             Orientation                                                                         Birefrin-                                                                          Temp.                                                                             Ratio                                                                             Temp.                                                                             Ratio                                                                             Ratio                                                                             Longi-                                                                            Trans-                (°C.)                                                                           (Time)                                                                            (°C.)                                                                      (Time)                                                                            Factor                                                                              gence                                                                              (°C.)                                                                      (Time)                                                                            (°C.)                                                                      (Time)                                                                            (Time)                                                                            tudinal                                                                           verse                                                                             Area              __________________________________________________________________________    Example                                                                       Nos.                                                                          1    125 2.05                                                                              113 2.32                                                                              0.81  0.038                                                                              105 2.8 150 1.7 1.7 8.08                                                                              4.76                                                                              38.5              2    125 2.05                                                                              113 2.32                                                                              0.81  0.038                                                                              105 2.8 150  1.85                                                                              1.85                                                                             8.30                                                                              5.04                                                                              45.6              3    125 2.05                                                                              113 2.32                                                                              0.81  0.038                                                                              105 2.8 150  2.05                                                                              2.05                                                                             9.75                                                                              5.74                                                                              56.0              4    125 2.05                                                                              113 2.32                                                                              0.81  0.038                                                                              105 2.8 150 1.5 2.2 7.13                                                                              6.16                                                                              43.9              5    125 2.05                                                                              113 2.32                                                                              0.81  0.038                                                                              105 2.8 150 2.1 1.8 9.98                                                                              5.04                                                                              50.3              6    130 1.8 115 2.37                                                                              0.69  0.029                                                                              105 3.0 150 1.7 1.7 7.52                                                                              5.10                                                                              37.0              7    130 1.8 115 2.37                                                                              0.69  0.029                                                                              105 3.0 150 1.8 1.8 7.68                                                                              5.40                                                                              41.5              8    120 2.05                                                                              110 2.54                                                                              0.92  0.068                                                                              105 3.2 160 1.5 1.5 7.81                                                                              4.80                                                                              37.5              9    120 2.05                                                                              110 2.54                                                                              0.92  0.068                                                                              105 3.2 160 1.6 1.6 8.33                                                                              5.12                                                                              42.7              10   130 1.88                                                                              113 2.4 0.76  0.031                                                                               95 3.0 160 1.8 1.8 8.12                                                                              5.40                                                                              43.9              11   130 1.88                                                                              113 2.4 0.76  0.031                                                                               95 3.0 160 2.0 2.0 9.02                                                                              6.00                                                                              54.1              Compar-                                                                       ative                                                                         Example                                                                       Nos.                                                                          1     93 4.0 --  --  0.91  0.16 105 3.0 150 1.4 1.4 5.60                                                                              4.20                                                                              23.5              2    105 3.0 --  --  0.42  0.061                                                                              105 3.0 150 1.8 1.8 5.40                                                                              5.40                                                                              29.2              3    130 4.0 --  --  0.26  0.013                                                                              105 3.0 150 1.4 1.4 5.60                                                                              4.20                                                                              29.2              4    100 1.8 100 2.3 0.84  0.124                                                                              105 3.0 150 1.4 1.4 5.80                                                                              4.20                                                                              24.3              5    100 1.8 130 2.3 0.46  0.071                                                                              105 3.0 150 1.5 1.5 6.21                                                                              4.50                                                                              27.9              6    130 1.7 113 1.5 0.52  0.018                                                                              105 3.0 150 1.8 1.8 4.59                                                                              5.40                                                                              24.8              __________________________________________________________________________

                                      TABLE 1-2                                   __________________________________________________________________________                                   Number of                                                                     times of                                       F - 5 value (kg/cm.sup.2)                                                                       Heat shrinkage (%)                                                                         breakage                                       Longitudinal                                                                              Transverse                                                                          Longitudinal                                                                         Transverse                                                                          (per 9,000 m)                                                                        Remarks                                 __________________________________________________________________________    Example                                                                       Nos.                                                                          1    2200   1720  1.33   0.56  0      --                                      2    2400   1700  1.62   0.63  0      --                                      3    2540   1750  1.79   0.74  1      --                                      4    1550   3180  -0.02  1.96  0      --                                      5    3240   1520  2.03   0.30  0      --                                      6    1860   1760  1.19   1.24  0      --                                      7    2190   2000  1.53   1.43  0      --                                      8    2270   1750  1.71   0.41  0      --                                      9    2520   1820  1.96   0.61  0      --                                      10   1690   2160  1.73   1.94  0      --                                      11   1780   2180  1.21   2.33  1      --                                      Compar-                                                                       ative                                                                         Example                                                                       Nos.                                                                          1    1250   1160  1.53   1.24  0      Frequent breakage at a                                                        simultaneous                                                                  biaxial stretching ratio of about                                             1.5.                                    2    1180   1320  1.33   1.43  8      Frequent breakage at a                                                        simultaneous                                                                  biaxial stretching ratio of about                                             1.8.                                    3    1430   1010  1.82   0.94  4      Frequent breakage at a                                                        simultaneous                                                                  biaxial stretching ratio of about                                             1.5.                                    4    1380   1090  1.44   0.86  0      Frequent breakage at a                                                        simultaneous                                                                  biaxial stretching ratio of about                                             1.5.                                    5    1740   1430  2.92   1.26  6      Frequent breakage at a                                                        simultaneous                                                                  biaxial stretching ratio of about                                             1.6.                                    6    1220   1340  1.14   1.60  3      Frequent breakage at a                                                        simultaneous                                                                  biaxial stretching ratio of about                                             1.8.                                    __________________________________________________________________________

EXAMPLES 12-17

Molten linear polyethylene terephthalate was extruded through a die sliton a drum cooled to a temperature below 80° C. to cool and solidify thesame. The obtained, substantially non-oriented film was stretched underconditions shown in Table 2-1 and then heat-set at 200° C. under tensionfor 6 sec. The final film thickness was adjusted to 8μ in all cases bycontrolling the amount of extrusion of the molten polymer depending onthe conditions.

Characteristic properties of the films obtained by the above-mentionedprocesses were as shown in Table 2-2.

A composition comprising the following components:

    ______________________________________                                        Ferromagnetic alloy powder (Fe--Co--B)                                                                  300 parts                                           (average particle size: 400 Å)                                            Zinc powder (average particle size: 2μ)                                                              25 parts                                            Cellulose acetate butyrate                                                                              30 parts                                            Epoxy resin               25 parts                                            Silicone oil (lubricant)  4 parts                                             Lecithin (dispersant)     5 parts                                             Toluene (solvent)         200 parts                                           Methyl ethyl ketone (solvent)                                                                           200 parts                                           Ethyl acetate (solvent)   100 parts                                           ______________________________________                                    

was charged in a ball mill and kneaded sufficiently. Then, 180 parts ofa polyisocyanate compound (Desmodur L-75) was added to the mixture andthe whole was stirred for 30 min. Then, the mixture was applied to onesurface of the polyethylene terephthalate film obtained as above in sucha manner that a coating thickness after drying of 3μ would be obtainedunder the application of magnetic field. The film was dried and thensubjected to the curing treatment and mirror finish. The film was slitin a width of 1/2 inch to obtain a video tape.

Characteristic properties of the obtained video tapes are shown in Table2-2.

In the column of load for 5% elongation in Table 2-2, symbols , and αrepresent that the results were quite good, good and not so good,respectively. It will be understood that films having a high F-5 valueare suitable for use as magnetic tapes.

A term "heat touch" indicates a state of contact of a magnetic tape withmagnetic head in a helical scan-type VTR. It has been known that thehead touch is reduced as the film thickness becomes bad.

In this Table, "A" represents that the head touch was good because theoutput signal voltage waveform by the touch in the tape playing wasstrong and flat when viewed as a whole picture. "B" represents that thetouch was not so good because the signal was distorted upwards ordownwards in its middle portion. "C" represents that the touch was badbecause the signal per se was weak and deformed. As seen from the Table,a thin base film of a thickness of 8μ used in the Examples of thepresent invention has a high F-5 value favorably. A low film strengthresulted in unstable running properties and disorder of the picturequality or even tape-stopping-behavior.

Further, when the films are heated in the step of drying the coatinglayer and curing step (vacuum metallizing step in case of a metallizedmagnetic tape), heat shrinkage of the film occurs to cause disorder ofthe picture quality. The stress is changed remarkably particularly atthe time of starting or stopping of the magnetic tape to invite thedisorder of the picture quality. The picture quality is shown bydisorder of the video picture quality of a running tape in three grades(, and α) and starting and stopping properties are shown by disorder ofthe picture quality at the times of starting and stopping in threegrades (, and α). The magnetic tape of the present invention is suitablefor use as a thin magnetic recording medium, since it has an excellenthead touch, mechanical strength and dimensional stability whichwithstand rough use, stable running properties and good electromagnetictransducing properties (Read/Write characteristics of magneticrecording).

COMPARATIVE EXAMPLES 7-11

For comparison, video tapes were produced under the same conditions asin Example 1 except that stretching conditions were the same as inComparative Examples shown in Table 1-1. Properties of the films for useas video tapes and properties of the video tapes were as shown in Table2-2.

The film obtained in Comparative Example 7 corresponds to a commerciallyavailable balance-type film. It had an insufficient tensile strength andwas unsuitable for use as the thin magnetic tape. In Comparative Example8, F-5 values of the film in longitudinal and transverse directions werewell-balanced so as to increase the F-5 value as far as possibleaccording to a known process. Head touch of this film was better thanthat of the film of Comparative Example 7 but it was still insufficient.In Comparative Examples 9 and 10, the films obtained were tensilizedfilms having an increased longitudinal restretching ratio. They had asignificantly deteriorated transverse F-5 value unfavorably. The filmobtained in Comparative Example 11 had in increased transverse F-5value. However, the film had an inferior longitudinal F-5 value and itwas unsatisfactory with respect to a load for 5% longitudinal elongationrequired of magnetic tapes.

                                      TABLE 2-1                                   __________________________________________________________________________                                               Simultaneous                                            Characteristics of    Biaxial                                                 films after multiple- Stretching                         Multiple Longitudinal                                                                              stage longitudinal                                                                      Transverse  Conditions                                                                            Total                      Stretching Conditions                                                                              stretching                                                                              Stretching  Longi-                                                                            Trans-                                                                            Stretching                 1st Stage    2nd Stage                                                                             Amorphous                                                                           Bire-                                                                             Conditions  tudinal                                                                           verse                                                                             Ratio (Time)               Temp.    Ratio                                                                             Temp.                                                                             Ratio                                                                             Orientation                                                                         frin-                                                                             Temp.                                                                             Ratio                                                                             Temp.                                                                             Ratio                                                                             Ratio                                                                             Longi-                                                                            Trans-                 (°C.)                                                                           (Time)                                                                            (°C.)                                                                      (Time)                                                                            Factor                                                                              gence                                                                             (°C.)                                                                      (Time)                                                                            (°C.)                                                                      (Time)                                                                            (Time)                                                                            tudinal                                                                           verse                                                                             Total              __________________________________________________________________________    Example                                                                       Nos.                                                                          12   125 2.05                                                                              113 2.32                                                                              0.81  0.038                                                                             105 2.8 150 1.85                                                                              1.85                                                                              8.80                                                                              5.04                                                                              45.6               13   130 1.8 115 2.37                                                                              0.69  0.029                                                                             105 3.0 150 1.8 1.8 7.68                                                                              5.40                                                                              41.5               14   130 1.88                                                                              113 2.4 0.76  0.031                                                                              95 3.0 160 2.0 2.0 9.02                                                                              6.00                                                                              54.1               15   125 2.05                                                                              113 2.37                                                                              0.81  0.038                                                                             105 2.8 150 2.05                                                                              2.05                                                                              9.96                                                                              5.74                                                                              57.2               16   130 1.88                                                                              113 2.40                                                                              0.76  0.031                                                                              95 3.0 160 1.85                                                                              1.85                                                                              8.35                                                                              5.55                                                                              46.3               17   125 2.05                                                                              113 2.32                                                                              0.81  0.038                                                                             105 2.8 150 1.5 2.2 7.13                                                                              6.16                                                                              43.9               Compar-                                                                       ative                                                                         Example                                                                       Nos.                                                                          7     93 4.0 --  --  0.91  0.16                                                                              105 3.0 150 1.4 1.4 5.60                                                                              4.20                                                                              23.5               8    130 1.7 113 1.5 0.30  0.018                                                                             105 3.0 150 1.8 1.8 4.59                                                                              5.40                                                                              24.8               9    130 4.0 --  --  0.26  0.013                                                                             105 3.0 150 1.4 1.4 5.60                                                                              4.20                                                                              29.2               10   100 1.8 100 2.3 0.65  0.080                                                                             110 3.5 150 1.4 --  5.80                                                                              3.50                                                                              20.3               11   100 1.8 100 2.3 0.65  0.080                                                                             110 3.5 150 --  1.4 4.14                                                                              4.90                                                                              20.3               __________________________________________________________________________

                                      TABLE 2-2                                   __________________________________________________________________________    F - 5 value  Heat shrinkage                                                   (kg/cm.sup.2)                                                                              (%)     Amorphous                                                                           Average crystal                                                                       Load for 5%    Starting and                Longi-   Trans-                                                                            Longi-                                                                            Trans-                                                                            orientation                                                                         particle size                                                                         elongation of                                                                        Head                                                                              Picture                                                                           stopping                    tudinal  verse                                                                             tudinal                                                                           verse                                                                             factor (-)                                                                          (Å) magnetic tape                                                                        touch                                                                             qualify                                                                           properties                  __________________________________________________________________________    Example                                                                       Nos.                                                                          12   2400                                                                              1820                                                                              1.60                                                                              0.63                                                                              0.31  39.6    ⊚                                                                     A   ⊚                                                                  ⊚            13   2200                                                                              2010                                                                              1.09                                                                              0.89                                                                              0.12  38.4    ⊚                                                                     A   ⊚                                                                  ⊚            14   1850                                                                              2180                                                                              0.94                                                                              1.03                                                                              -0.21 38.8    ⊚                                                                     A   ⊚                                                                  ⊚            15   2860                                                                              1630                                                                              2.05                                                                              0.36                                                                              0.43  43.2    ⊚                                                                     A   ⊚                                                                  ⊚            16   1750                                                                              2430                                                                              0.62                                                                              1.62                                                                              -0.63 38.3    ○                                                                             A   ⊚                                                                  ⊚            17   1600                                                                              2800                                                                              0.43                                                                              1.91                                                                              -1.22 32.6    ○                                                                             A   ⊚                                                                  ⊚            Compar-                                                                       ative                                                                         Example                                                                       Nos.                                                                          7    1210                                                                              1150                                                                              3.4 0.44                                                                              0.22  53.7    Δ                                                                              C   Δ                                                                           Δ                     8    1460                                                                              1410                                                                              2.1 0.88                                                                              0.21  51.2    Δ                                                                              B   ⊚                                                                  Δ                     9    1820                                                                              1020                                                                              2.8 0.91                                                                              0.43  52.1    Δ                                                                              C   ○                                                                          ○                    10   1850                                                                               980                                                                              4.4 0.76                                                                              0.51  55.7    Δ                                                                              C   Δ                                                                           ○                    11   1020                                                                              1760                                                                              3.2 2.9 0.21  53.6    Δ                                                                              A   Δ                                                                           Δ                     __________________________________________________________________________

What is claimed is:
 1. A magnetic recording medium comprising apolyethylene terephthalate film having the sum of F-5 values inlongitudinal and transverse directions of 3500 to 5000 kg/cm², absoluteheat shrinkage in longitudinal and transverse directions at 100° C. ofup to 2.5% and an amorphous orientation factor of -1.5 to 0.5 andmagnetic layer(s) formed on at least one surface of the film,said filmbeing obtained by subjecting a polyethylene terephthalate sheet tomulti-stage longitudinal stretching, transverse stretching and biaxialrestretching, wherein the total stretching ratio is at least
 27. 2. Amagnetic recording medium according to claim 1 wherein the magneticrecording medium is a magnetic recording tape or magnetic recordingdisc.
 3. A magnetic recording medium according to claim 1 wherein themagnetic layer comprises at least one member of the group consisting ofγ-Fe₂ O₃, Co-doped γ-Fe₂ O₃, CrO₂, Fe, Co, Ni and a mixture of at leastone of them with a binder.
 4. A magnetic recording medium according toclaim 1 wherein the magnetic layer is formed by a method selected fromthe group consisting of vacuum metallizing, sputtering, ion plating,electroplating and coating methods.
 5. A magnetic recording mediumaccording to claim 1 wherein the magnetic layer has a thickness of 0.05to 5μ.
 6. A magnetic recording medium according to claim 1 wherein themagnetic recording medium has a thickness of 4 to 9μ.
 7. A magneticrecording medium according to claim 1 wherein the magnetic recordingmedium is a video tape.
 8. A magnetic recording medium according toclaim 1 wherein the magnetic recording medium is a computer tape.
 9. Amagnetic recording medium according to claim 1 wherein the magneticrecording medium is an audio tape.